PTFE porous bodies are excellent in heat resistance and chemical resistance as well as in electrical properties, such as relative dielectric constant and energy loss angle, and thus are used for multiple purposes, such as electric wire covering materials, dielectric materials for coaxial cables, filters, gaskets, heat insulating materials, separation membranes, artificial blood vessels, catheters, and incubators.
In a well-known method for producing such a PTFE porous body, a mixture of a PTFE powder and a binding agent is fine ground and molded into a molding by a known method, and then the molding is baked.
In another well-known method for producing a PTFE porous body, a mixture of a PTFE powder and a pore-forming agent is molded into a predetermined shape, and then the pore-forming agent is removed to make pores.
Patent Document 1, for example, discloses a method for producing a PTFE porous body by baking a non-baked PTFE at a temperature higher than a melting point of PTFE, grinding the baked PTFE to provide a baked PTFE powder, and subsequently molding the powder into a molding having a predetermined shape by a pressure of 1 g/cm2 to 800 kg/cm2 and again baking the molding at a temperature higher than the melting point of PTFE.
Patent Document 2, for example, discloses a method for producing a PTFE porous body which includes a step of mixing a PTFE powder and a binding agent having a lower melting point than PTFE and a decomposition temperature higher than a baking temperature of PTFE, a step of gelling and then fine grinding a resulting mixture, a step of performing ram extrusion of a fine ground powder to make a preform, and a step of baking the preform under unrestrained condition.
Patent Document 3, for example, discloses a method for producing a porous body by molding a PTFE including a liquid lubricant, which acts as a pore-forming agent, and heating a molded PTFE in a stretched state. Also, there is disclosed, as a conventional technology, a method for producing a porous body by mixing a PTFE and a liquid lubricant which acts as pore-forming agents, molding a mixture, and then removing the liquid lubricant. Examples of the liquid lubricant listed therein are naphtha, white oil, toluole, xylole, and the like.
Patent Document 4 discloses a method for producing a porous body by molding a mixture of a PTFE powder, and a foaming agent and a liquid lubricant, which act as pore-forming agents, into a predetermined shape, heating a molded mixture to cause foaming to thereby making countless fine pores, and subsequently performing stretching. Examples of the foaming agent listed therein are azo foaming agents, hydrazide foaming agents, semicarbazide foaming agents, nitroso foaming agents, ammonium carbonates, sodium bicarbonates, ammonium nitrites, and the like. Examples of the liquid lubricant listed therein are liquid paraffin, naphtha, white oil, toluene, xylene, and the like.
Patent Document 5 discloses a producing method of mixing of a PTFE powder with a fine pore-forming agent, an expanding agent, and a lubricant oil, which act as pore-forming agents; cold extrusion; sequentially performing evaporation of the lubricant oil, sublimation or decomposition of the fine pore-forming agent and the expanding agent; and baking of PTFE. An example of the lubricant oil therein is a mixture of aliphatic hydrocarbon. Examples of the fine pore-forming agent listed therein are compounds, such as benzene, toluene, naphthalene, benzaldehyde, and aniline, or monohalide or polyhalide derivatives of these compounds. Examples of the expanding agent listed therein are azodicarbonamide, modified azodicarbonamide, 5-phenyltetrazole and derivatives thereof, and aromatic derivatives of hydrazine.
Patent Documents 6 and 7 disclose heating and baking a PTFE including a pore-forming agent to make the PTFE porous by the action of the pore-forming agent. Examples of the pore-forming agent listed therein are ammonium hydrogen carbonate, ammonium carbonate, and ammonium nitrite.
Patent Document 8, discloses a method for producing a porous body by performing extrusion of a PTFE including a foaming agent, which acts as a pore-forming agent, and subsequently removing the foaming agent. Examples of the foaming agent listed therein are azo compounds, sodium carbonate, ammonium carbonate, hydrazine, tetrazole, benzoxazine, semicarbazide, and the like.
Patent Document 9 discloses combining camphor, menthol, and naphtha appropriately for use as a pore-forming agent.    Patent Document Japanese Unexamined Patent Publication No. 61-66730    Patent Document 2: Japanese Unexamined Patent Publication No. 5-93086    Patent Document 3: Japanese Examined Patent Publication No. 42-13560    Patent Document 4: Japanese Examined Patent Publication No. 57-30059    Patent Document 5: Japanese Unexamined Patent Publication No. 60-93709    Patent Document 6: Japanese Unexamined Patent Publication No. 11-124458    Patent Document 7: Japanese Unexamined Patent Publication No. 2001-67944    Patent Document 8: Japanese Unexamined Patent Publication No. 2004-500261    Patent Document 9: Japanese Unexamined Patent Publication No. 2005-336459